Talk Abstract:
Submesoscale processes provide a pathway for energy to transfer from the balanced circulation to turbulent dissipation. One class of submesoscale phenomena that has been shown to be quite effective at mixing and removing energy from the balanced flow are centrifugal-symmetric instabilities (CSIs), which have been observed to generate significant mixing in both the surface boundary layer and bottom boundary layer flows along bathymetry, where they have been implicated in the mixing and watermass transformation of Antarctic Bottom Water. However, the mixing efficiencies (i.e. the fraction of the energy extracted from the flow used to irreversibly mix the fluid) of these instabilities remain uncertain, making estimates of global mixing and energy dissipation due to CSI difficult.
Thus, in this work we use large-eddy simulations to investigate the mixing efficiencies of CSIs in the submesoscale range. We find that CSIs that are centrifugally-dominated (i.e. mostly driven by shear production) tended to have higher mixing efficiencies than symmetrically-dominated ones (i.e. driven by vertical shear production). Mixing efficiencies in CSIs can therefore alternately be significantly higher or significantly lower than the canonical value used by most studies (≈0.17) depending on the dominant source of shear production. These results can be understood in light of recent work on stratified turbulence, whereby CSIs control the background state of the flow in which smaller-scale secondary overturning instabilities develop, thus actively modifying the characteristics of mixing by Kelvin-Helmholtz instabilities. Our results also suggest that it may be possible to predict these efficiencies with measured properties of the flow (namely the Richardson and Rossby numbers), possibly allowing for parameterization of this effect.
